1. Field of the Invention
This invention generally relates to semiconductor fabrication processes and, more particularly, to a mask and corresponding process for crystallizing the mask slit end areas associated with lateral growth crystallization.
2. Description of the Related Art
When forming thin film transistors (TFTs) for use in liquid crystal display (LCD) or other microelectronic circuits, the location of transistors channel regions, the orientation of regular structured polycrystalline silicon (poly-Si) or single-grain-crystalline silicon, and the surface roughness are important issues. This poly-Si material can be used as the active layer of poly-Si TFTs in the fabrication of active-matrix (AM) backplanes. Such backplanes can be used in the fabrication of AM LCDs and can be also combined with other display technologies, such as organic light-emitting diode (OLED) displays.
Poly-Si material is typically formed by the crystallization of initially deposited amorphous Si (a-Si) films. This process can be accomplished via solid-phase-crystallization (SPC), for example, by annealing the a-Si films in a furnace at appropriate temperature and for sufficiently long time. Alternatively, laser annealing can also be used to achieve the phase transformation.
Conventionally, crystallization techniques are applied to a substrate in such a manner as to yield uniform poly-Si film quality throughout the substrate area. In other words, there is no spatial quality differentiation over the area of the substrate. The most important reason for this end result is the inability of conventional methods to achieve such quality differentiation. For example, when a-Si film is annealed in a furnace or by rapid-thermal-annealing, all of the film is exposed to the same temperature, resulting in the same quality of poly-Si material. In the case of conventional laser annealing, some differentiation is possible, but the price, in terms of loss of throughput, is very high for the modest performance gains realized.
Recently, a new laser annealing technique has been developed that allows for significant flexibility in the process techniques, permitting controlled variation in resulting film microstructure. This technique relies on lateral growth of Si grains using very narrow laser beams that are generated by passing a laser beam through a beam-shaping mask, and projecting the image of the mask to the film that is being annealed. The method is called Laser-Induced Lateral Crystallization (LILaC), sequential lateral solidification (SLS), or SLS/LILAC.
Conventional solid state (SSL) or continuous laser annealing processes, can be differentiated from LILAC processes by their use of relatively rapid repetition rates, on the order of 10 to 100 kHz, whereas LILaC processes typically use an Excimer laser with a repetition rate that rarely exceeds 300 Hz. Further, SSL processes cannot typically use a beam shaping mask, since a more strongly coherent light is sourced. The poly-Si material crystallized by the SSL or continuous laser annealing method consists of a large density of grains, and each grain is surrounded by grain boundary. The size of grains are typically ˜1 micron (micrometer or μm). But the typical channel length of TFT is 2-30 microns, so it is inevitable that channel regions of TFT contain several grain boundaries. These grain boundaries act as electron and hole traps, and degrade the TFT characteristics and reliability. The LILAC process can form larger grain lengths between grain boundaries.
FIG. 1 is a diagram illustrating the LILaC process (prior art).
FIG. 2 illustrates a conventional LILaC beam shaping mask (prior art). Referencing FIGS. 1 and 2, the initially amorphous silicon film is irradiated by a very narrow laser beamlet, with typical widths of a few microns (i.e. 3-5 μm). Such small beamlets are formed by passing the original laser beam through a mask that has open spaces or apertures, and projecting the beamlets onto the surface of the annealed Si-film.
The sequence of images 1 through 4 illustrates the growth of long silicon grains. A step-and-repeat approach is used. The shaped laser “beamlet” (indicated by the 2 parallel, heavy black lines) irradiates the film and then steps by a distance smaller than half of the width of the slit. As a result of this deliberate advancement of each beamlet, grains are allowed to grow laterally from the crystal seeds of the poly-Si material formed in the previous step. This is equivalent to laterally “pulling” the crystals, as in zone-melting-crystallization (ZMR) method or other similar processes. As a result, the crystal tends to attain very high quality along the “pulling” direction, in the direction of the advancing beamlets. This process occurs simultaneously at each slit on the mask, allowing for rapid crystallization of the area covered by the projection of the mask on the substrate. Once this area is crystallized, the substrate moves to a new (unannealed) location and the process is repeated.
To control the grain boundary pulling, conventional LILaC apertures widths have been limited to no greater than 4 to 5 microns. When the aperture is greater than about 4 microns, the Si area furthest from the growing crystal seed spontaneously crystallizes in an undesirable pattern. That is, the slit width is limited by the lateral growth length.
Some poly-Si materials formed through the LILaC process have a highly periodical microstructure, where crystal bands of specific width are separated by high-angle grain boundaries. Within the crystal bands, low-angle boundaries are observed with a frequency of occurrence dependent upon certain specifics of the crystallization process, such as film thickness, laser fluence (energy density), pulse duration, and the like. TFTs fabricated on such poly-Si films demonstrate very good characteristics, as long as the direction of conduction is parallel to the direction of the in-crystal low-angle boundaries.
TFTs with greater electron mobility can be fabricated if the substrate crystallization characteristics can be made more isotropic. In other words, the TFT performance depends upon the angle between the main crystalline growth direction, the direction parallel to the laser scanning axis, and the TFT channel. This is due to the formation of sub-boundaries within the crystal domains. Therefore, by chance only, depending upon the relative size of the crystal domain and the TFT channel length, certain TFTs will not include grain-boundaries in their active area (channel), whereas other TFTs will include one or more boundaries in their active areas. This kind of non-uniformity is highly detrimental for critical-application TFTs where uniformity of characteristics is more essential than absolute performance.
If the angle of rotation between the lattice mismatch on the two sides of the boundary is less than approximately 15 degrees, the boundary is considered to be a low-angle boundary. An angle of rotation between 15 and 90 degrees is considered to be a high-angle boundary. Electron mobility between high-angle boundaries is impaired, while mobility between low-angle boundaries is usually insignificant. The step-and-repeat annealing typically promotes low-angle boundaries. However, the film regions corresponding to the mask edges, not being subject to the step-and-repeat process, are likely to form high-angle boundaries.
One embodiment of the SLS/LILaC process involves the use of a large array of narrow slits that simultaneously melt and solidify the Si thin film in such a way as to fully crystallize the entire film after two passes, stitching together crystallized strips. The mask, used to shape the beam, can in principal have a wide variety of patterns on it. The mask may comprise a patterned layer of chrome, or other material that blocks the desired wavelength effectively, on a quartz substrate, or other suitably transparent material at the wavelength of laser to be used. Common patterns consist of groups of rectangular shapes, including slits and chevrons.
As the laser beam is projected through these patterns, the intensity profile of the projected beam is determined by the features that make up the pattern and any optics used to image the pattern on the material. The intensity profile of the laser beam is typically not uniform over the entire pattern. For example, at corner regions intensity peaks have been noticed. These intensity peaks may cause local damage on the film irradiated by the shaped beam. One form of damage caused by intensity peaks is agglomeration, which may cause the silicon film to pull away from the region exposed to the high intensity peaks, possibly leaving a void or other non-uniformity.
One area of non-uniformity is associated with the mask slit ends. Even if the Si in the area underlying the slit ends is not damaged, the pattern of crystallization and grain boundaries are not as controlled as in the other slit areas. Thus, high performance TFTs cannot be fabricated in these slit end Si areas having uncontrolled crystallization boundaries.
It would be advantageous if a Si film could be uniformly crystallized using a lateral growth process, even in the areas underlying the mask slit ends.
It would be advantageous if a mask slit end could be modified to produce uniform and controlled grain boundaries.
It would be advantageous if the slit widths of lateral growth masks could be increased to improve the rate of Si film crystallization. It would be advantageous if the above-mentioned modified mask slit end could be adapted for use with a wide slit mask.